The present invention relates to bandwidth optimization and efficient processor utilization in a cellular telephone system, and more particularly to bandwidth optimization and efficient processor utilization in remote or mobile station paging in a cellular telephone system.
Interim Standard (IS) 136 (promulgated by the Telecommunications Industry Association) adds a Digital Control Channel (DCCH) to IS-54B, an 800 MHz TDMA Cellular Standard. The fundamental unit of measure in the digital control channel is a slot. Slots are logically combined (in groups of 16 for a half-rate DCCH and 32 for a full-rate DCCH) to form "Superframes".
Like the Japanese and European digital cellular standards, IS-136, which in early 1995 will replace IS54B as the new North American standard for 800 MHz dual mode TDMA cellular telephony, was specified as a "layered" standard. The theory behind layering standards is discussed in detail in the CCITT (Blue Book) Recommendation X.200. The following paragraphs give a brief introduction to the philosophy behind the layering used in the IS-136 standard.
At the highest layer of control in the air interface, complex state machines (e.g., Call Control, Mobility Management) are reacting to external events and incoming message traffic by generating outgoing "Messages" (an Origination Message, for example) according to the IS-136 standard. On example of such layering is depicted in FIG. 1. Since state machines that exist in the mobile station and the base station are complimentary, the depiction in FIG. 1 is representative of the protocol machine implemented in either side of the air interface (i.e., cellular communications channel).
Messages generated by "layer 3" (as depicted in FIG. 1) state machine processing are sent to "layer 2" for segmentization into one or more "Frames". Since many messages can not fit entirely into a single Frame, these Messages must be spread over several Frames according to rules specified for layer 2 processing in IS-136. There are several types of layer 2 Frames, and the type used depends upon the layer 3 Message being framed.
Frames produced by layer 2 are passed to a "layer 1" (or physical layer) for insertion into Slots, encoding, interleaving, modulation, and ultimately transmission over the air. Each type of Frame is designed to fit exactly into a single Slot. Therefore, there is no further decomposition of the layer 3 Message.
The decomposition process associated with sending a Message is symmetric with the process of recomposition a Message that is received. In addition, these protocols are considered "peer-to-peer", meaning that there is an implied connection between the layer 2 at the mobile station and the layer 2 at the base station, and a similar implied connection between their layer 1's and layer 3's. Thus, a Frame assembled by the layer 2 in the mobile station is disassembled by the layer 2 in the base station, and each is theoretically unaware of the means by which the Frame was carried between them, i.e., their respective layer 1's.
When a base station receives notification (from the mobile telephone switching office (MTSO)) of an incoming call, it must locate the mobile station in order to notify the mobile station of the incoming call. To do this, the base station sends a Page Message over any digital control channel (DCCH) that the mobile station may be monitoring, which generally includes DCCH's within several cells. If the mobile station responds to the Page Message with a Page Response, the base station is notified that the mobile station has been successfully located. The base station responds to the Page Response by proceeding with the rest of call setup, as specified in IS-136.
The mobile unit, however, is not always monitoring the digital control channel (DCCH) for Page Messages. Each Slot in a given digital control channel has a specified function (in accordance with IS-136). Some, called broadcast control channel (BCCH) slots, are allocated to carry overhead information to all mobile stations that may be monitoring a particular digital control channel (point-to-many). Others, called Short Message Service, Paging, and Access Response Channel (SPACH) slots, are used by the base station to carry point-to-point Messages to a specific mobile station. Together the BCCH and the SPACH (as possibly some Reserved slots) comprise a Superframe. Each Superframe is made up of a total of thirty-two slots (in a full-rate DCCH), each allocated as a BCCH slot or a SPACH slot (or possibly a Reserved slot). Superframes are transmitted in pairs referred to as Primary and Secondary Superframes, respectively. In a quiescent system, the mobile station is only required to monitor a single SPACH slot (called its Paging Channel (PCH) Subchannel) in every other Superframe, i.e., in every Primary Superframe. During the period between occurrences of its PCH Subchannel, the mobile station may shut off its Transmitter, Receiver, and possibly even some of its processors. This "shut off" state, is referred to herein as a "sleep mode," and accounts for a significant reduction in power consumption by the mobile unit, which results in increased battery life in battery powered mobile units.
In mature, i.e., highly utilized, IS-136 systems, many mobile stations are assigned to the same PCH Subchannel, and there are (potentially frequent) occasions where more than one of the mobile stations assigned to a given PCH Subchannel will have to be simultaneously paged by the base station.
Heretofore, there have been several techniques employed for sending pages to more than one mobile station in a single PCH Subchannel on a given digital control channel. One technique is to construct a layer 3 Page Message including up to four 34-bit Mobile Identification Numbers (MIN) in the Page Message. This technique, however, involves layer 3 processing at both the mobile station and the base station (which necessarily implies high processor utilization), and more than one layer 2 Frame may be required to send the message, (which implies more than one layer 1 SPACH slot is required). This technique therefore does not efficiently utilize processing resources or bandwidth.
A second technique for paging more than one mobile station in a single PCH Subchannel slot has been to use a "Hard Page", which is a special type of layer 2 Frame that carries three, four or five different mobile station addresses, but no layer 3 Message information. This serves to not only reduce layer 3 involvement, thereby reducing processing overhead, but is also more bandwidth efficient, because a Hard Page Frame fits into a single PCH Subchannel slot. Thus, of the techniques available to the base station for paging more than one mobile station in a slot, the most efficient are "Hard Pages". Because Hard Pages carry only address information for the mobile stations being paged and because "Hard Pages" consume only a single slot, they are very efficient from both a bandwidth and a processing standpoint.
Unfortunately however, Hard Pages are designed to carry exactly three 34-bit Mobile Identification Numbers (MIN) for the purposes of paging exactly 3 mobile stations, to carry four 24-bit Temporary Mobile Station Identifiers (TMSIs) to page exactly four mobile stations, or to carry four or five 20-bit TMSIs to page exactly four or five mobile stations, respectively.
It is possible, however, that only one or two mobile stations must be paged in a single slot, thereby precluding the use of a Hard Page. When paging a single mobile station using a 34-bit MIN, the base station can format a Page Message (not a Hard Page) and still use only a single slot. However, the mobile station must decode the Page Message, because it is a layer 3 message, in order to ascertain that it is a page. This is less efficient than the processing required for a Hard Page, which is done almost completely at layer 2. When paging two mobile stations using 34-bit MINs, an entire Page Message does not fit into a single slot. Therefore, in addition to bearing the overhead of processing a layer 3 Page Message, two slots are consumed in order to page two mobile stations, thereby consuming additional bandwidth.